Semi-solid metal casting process

ABSTRACT

A method of semi-solid metal casting without the need for retrofitting of conventional casting equipment is provided. A method of direct-feed casting of semi-solid metals is also provided.

FIELD OF THE INVENTION

The present invention relates generally to the field of processes forcasting metal alloys. More particularly, the present invention relatesto a process and apparatus for semi-solid metal casting of metal alloys.

BACKGROUND OF THE INVENTION

Regular casting methods such as conventional die casting, gravitypermanent mold casting, and squeeze casting have long been used formetals and their alloys. However, these current processes when used tomanufacture parts with relatively complex geometries often yieldproducts with undesirable shrink porosity, which can adversely impactthe quality and integrity of the part. Shrink porosity defines acondition that arises as a metal part begins to shrink as it cools andsolidifies along the outer surface, leaving pockets of air (referred toas “voids”) trapped in the center of the part. If the voids are notreconstituted with metal, the cast part is termed “porous”. Particularlyin the design of complex parts, such as, for example, automotivetransmission valve bodies or engine bedplates, the greatest shrinkporosity is found in the thicker areas.

One method of reducing shrink porosity is to cast semi-solid metal (SSM)instead of liquid molten metal. SSM casting, which generally involveslow temperature, low velocity, and less turbulent injection of metal,typically reduces the occurrence of shrink porosity. Where SSM castingof metal materials has been involved however, the conventional methodshave not been employed successfully to date. Rheocasting andthixocasting are casting methods that were developed in an attempt toconvert conventional casting means to SSM casting, but these SSM methodsrequire costly retrofitting to conventional casting machinery andattempts at conventional casting of SSM have been unsuccessful.

Another method to reduce porosity levels is to apply a direct-feedsystem. The direct-feed system allows molten metal to continue to feeddirectly into the areas of thick geometry during solidification, therebyfilling the air pockets with metal as they form. In this way, shrinkporosity can be significantly reduced in those areas. Preferably, thedirect feed can be localized to multiple areas within particularlycomplex parts or as required.

Accordingly, it is desirable to provide a method of casting SSM metalsand alloys utilizing conventional and/or rheocasting die casting devicesthat can impart desirable mechanical properties. It is further desirableto provide a process to control the shrink porosity of cast parts atmultiple locations though out a part. Further still, it is desirable toprovide a method of producing products with metal alloy castings whereinthe temperature of the semi-solid metal slurry can be controlled.

SUMMARY OF THE INVENTION

The foregoing needs are met, to an extent, by the present invention,wherein in one embodiment a process and an apparatus is provided thatenables the use of conventional die casting machinery in SSM casting.

In accordance with one embodiment of the present invention, a semi-solidmetal casting process is provided, comprising heating a metal alloy to achosen temperature, cooling the metal alloy for a determined period oftime to form a semi-solid metal, wherein the time can be zero, and thencasting the semi-solid metal in a vertical die casting machine. Themetal alloys of the invention can be aluminum-silicon alloys, includinghypoeutectic alloys and hypereutectic alloys. The vertical die castingmachines may also be an indexing-type machine. In embodiments where anindexing type vertical die casting machine is used, the time required toindex from the pour station to a transfer station may be chosen so thatthe metal alloy within will form a semi-solid slurry.

In accordance with another embodiment of the present invention, an SSMcast product that is manufactured by casting a metal alloy in an SSMcasting process using a vertical die casting machine is provided. Thevertical die casting machine may optionally include an indexing featuredescribed herein. The product may be manufactured by heating a metalalloy to a chosen temperature and then cooling the metal for adetermined period of time to form a semi-solid metal, wherein the timecan be zero, and then casting the semi-solid metal alloy in the verticaldie casting machine. The metal alloy may be an aluminum-silicon basedalloy, including hypoeutectic and hypereutectic alloys.

There has thus been outlined, rather broadly, certain embodiments of theinvention in order that the detailed description thereof herein may bebetter understood, and in order that the present contribution to the artmay be better appreciated. There are, of course, additional embodimentsof the invention that will be described below and which will form thesubject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of embodiments inaddition to those described and of being practiced and carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein, as well as the abstract, are for thepurpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conceptionupon which this disclosure is based may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross section of an exemplary vertical die castingpress of a type suitable for carrying out the functions of an embodimentof the invention.

FIG. 2 is a perspective view of an exemplary vertical die casting pressof a type suitable for carrying out the functions of an embodiment ofthe invention.

FIG. 3 is a front view of a mold with multiple “gates” in oneembodiment.

DETAILED DESCRIPTION

The invention will now be described with reference to the drawingfigures, in which like reference numerals refer to like partsthroughout. An embodiment in accordance with the present inventionprovides a method of SSM casting without the need for retrofitting ofconventional casting equipment. Moreover, other embodiments of theinstant invention provide a direct-feed semi-solid casting process.

In one embodiment, vertical die casting machines or presses of thegeneral type disclosed in U.S. Pat. Nos. 5,660,233 and 5,429,175,assigned to and commercially available from THT Presses, Inc., Dayton,Ohio, are desirable. The THrr presses such as a 200 Ton Indexing ShotMachine, a 1000 Ton Shuttle Machine or a 100 Ton Shuttle Machine, inparticular, are capable of operating at a higher speed and with ashorter cycle time than previously known die casting presses and which,as a result, produce higher quality parts without porosity. The diecasting presses are also simpler and less expensive in construction,requiring less maintenance and therefore more convenient to service.

One of ordinary skill in the art will appreciate from the descriptionsherein, that some or all of the features of the presses of the instantinvention may differ to some extent from those specified below dependingon the specific press, but that variations are to be expected and arewithin the scope and spirit of the present invention. By way of example,the TTF presses of this invention may be classified as “indexing-type”or “shuttle-type.” Though the indexing press will be detailed in anembodiment below, both types of presses may be used in the instantinvention.

Referring now to FIGS. 1 and 2, in accordance with one embodiment of thepresent invention, a vertical die casting press 10 includes a frame 20having a base 30 supporting a vertical pedestal 40 or post on which ismounted a rotary indexing table 50. The table 50 supports a pair ofdiametrically opposite shot sleeves 60 each of which receives a shotpiston 65 connected to a downwardly projecting piston rod 67. A gateplate 90 extends horizontally between the side walls of the frame 20 andabove the indexing table 50 for supporting a lower mold 70 sectiondefining a cavity 61. When the table 50 is indexed in steps of 180degrees, the shot sleeves 60 are alternately located at a metalreceiving or pour station 80 and a metal injecting or transfer station85 under the gate plate 90. A hydraulic clamping cylinder 100 issupported by the frame 20 above the transfer station 85 and moves anupper mold 110 section vertically above the lower mold 70 section.

A high pressure hydraulic shot cylinder 120 is mounted on the base underthe transfer station 85, and a substantially smaller hydraulic ejectioncylinder 130 is mounted on the base 30 under the metal receiving or pourstation 70. Each of the hydraulic cylinders 120 and 130 has anon-rotating vertical piston rod 121 and 131 which carries a set ofspaced coupling plates 140. Each set of plates 140 defines laterallyextending and opposing undercut grooves for slidably receiving anoutwardly projecting bottom flange on each of the shot piston rods. Thuswhen the rotary table 50 is indexed, the shot piston rods rotate withthe shot sleeves 60 and alternately engage the piston rods of the twofixed hydraulic shot 120 and ejection cylinders 130.

The upper platen moves downwardly to close and clamp the upper mold 110against the lower mold 70 or against a cavity defining part P confinedbetween the upper and lower molds 110 and 70. The hydraulic shotcylinder 120 is actuated for transferring the molten metal from eachshot cylinder 60 upwardly into the cavity 61 defined by the clamped moldsections 70 and 110. The cavity 61 is evacuated, and the shot piston 65is forced upwardly to inject the molten metal into the mold cavity orcavities. The molds 70 and 110 and the shot piston 65 are then cooled,optionally by circulating water through passages within the molds andshot piston, to solidify the die cast material. The shot cylinder 120then retracts the connected sprues 150 or biscuit downwardly into theshot sleeve 60 after the metal has partially solidified within the gateplate 90. After the table 50 is indexed 180 degrees, the smallerhydraulic ejection cylinder 130 is actuated for ejecting the biscuitupwardly to the top of the indexing table 50 where the biscuit isdischarged. The cycle is then repeated for die casting another part orset of parts.

In operation of the vertical die casting machine or press describedabove in connection with FIG. 1, a predetermined charge or shot ofmolten metal is poured into the shot sleeve 60 in the pour station 80.The shot sleeves 60 can be equipped with heaters and temperature sensorsto heat and or cool the metal as is desirable at any time, including theperiod while table 50 indexes 180 degrees. The lateral transfer of themolten metal and the upward injection of the metal into the moldcavities is also effective to degas the molten metal, thereby minimizingporosity of the solidified die cast parts. Preferably, a light suctionis applied to the cavities 108 and runner 202 and the injecting chamber146 to remove air from the chamber and to remove the gas separated fromthe molten metal within the shot cylinder.

It has now been found that the above described press can also be usedfor SSM casting. The use of metal slurry metal over molten metal reducesfluid turbulence when injected into the die, which also reduces theamount of air that may be trapped in the final part. Less air in thefinal part lends greater mechanical integrity and allows cast productsto be heat treated. In addition, metals that are SSM casting requiresless heat thereby reducing cost and improving longevity of the molds anddies.

Without being limited to or bound by theory, the microstructure of SSMcast products can determine the mechanical properties of the product.Moreover, it is understood by those of ordinary skill in the art thatthe microstructure can be manipulated prior to casting. One way tomanipulate the final microscructure of an SSM cast part is to control,thereby reduce, the time the metal remains in the SSM range. The pressesdescribed above afford such an opportunity. Specifically, the indexingtime (i.e., the delay between indexing between the pour station 80 andtransfer station 85) can be used to control the time the molten metal iscooled in the shot sleeve to reach the SSM range. That is, the amount oftime the metal spends in the shot sleeve before it is injected into themolds can be regulated or optimized for a desirable microstructure.Alternatively, molten metal at a predetermined temperature may be pouredinto the shot sleeve of shuttle presses, i.e. presses that lack theindexing feature.

Many metals and alloys known in the art can be used for SSM casting andcan be employed with the instant invention. In some embodimentsaluminum-silicon alloys can be used. By definition, aluminum alloys withup to but less than about 11.7 weight percent Si are defined“hypoeutectic”, whereas those with greater than about 11.7 weightpercent Si are defined “hypereutectic”. In all instances, the term“about” has been incorporated in this disclosure to account for theinherent inaccuracies associated with measuring chemical weights andmeasurements known and present in the art. In yet other embodiments,aluminum-silicon copper alloys and/or aluminum-copper alloys may be usedwith the present invention.

Preferably, the metal is be cast is heated in a range from about 10° C.to about 15° C. above the liquidus temperature (i.e., the semi-solidtemperature). For Al—Si alloys this generally ranges from about 585° C.to about 590° C. The melt temperature is then allowed to cool to form asemi-solid slurry before it is finally cast.

In one embodiment, a 380 alloy, (Al—Si—Cu alloy commonly used in theart) is heated to 590 to 595° C. Once heated to the desired temperature,the metal is then transferred to the shot sleeve 60 in the pour station85. The metal is then indexed to the transfer station 80, taking about 2seconds. During that period, the metal is cooled to between 585° C. and590° C. before being cast.

The optimal transfer time from the pour station 80 to transfer station85 can be experimentally determined and will vary depending on the metalor alloy being cast. Generally, for Al—Si alloys, a transfer time in theranging from about 0.5 seconds to about 5 seconds is preferred. In otherembodiments, the time may range from about 1 seconds to about 30seconds.

FIG. 3 exemplifies a gate plate that may be used in accordance with theinvention. As mentioned above, gated plates allow for direct feed ofmetal to multiple locations within a part simultaneously. Especiallytrue in complex parts, direct feed enables metal to be selectivelyinjected into specific locations within a part as the part cools. As apart cools and contracts along the edges, voids emerge within the centeror thicker portions of a cast. With gated plates, however, the potentialvoids may be continually supplied with metal so as to reduce thelikelihood of their emergence and thereby reduce porosity.

The present invention may be applied to cast a variety of parts known inthe art and all such applications are within the scope of the presentinvention. In a preferred embodiment of the present invention, the diecast process described herein is used to cast parts with relativelycomplex geometries. Such parts may include automotive parts, forexample, suspension components including knuckles and control arms, bedplates, swashplates, air conditioning compressor pistons, engine valvebodies and pump housings.

The present invention may be preferable suited for complex parts in thatthe presses described herein have a smaller ratio of upper die to lowerdie parting position than found in conventional die casting presseswhich can reduce the gas content in the part. Also, where inconventional die casting processes the dwell time is controlled bybiscuit thickness, the ingate controls dwell time in the presentinvention. The smaller ingates have smaller volumes to be cooled, andthereby solidification time is reduced. The casting process describedherein also requires less clamping force than required by other castingprocesses such as with high pressure die casting and/or squeeze casting.Moreover, the present invention employs a large number of cavities whichallows for more parts to be produced per given amount of time.

The many features and advantages of the invention are apparent from thedetailed specification, and thus, it is intended by the appended claimsto cover all such features and advantages of the invention which fallwithin the true spirit and scope of the invention. Further, sincenumerous modifications and variations will readily occur to thoseskilled in the art, it is not desired to limit the invention to theexact construction and operation illustrated and described, andaccordingly, all suitable modifications and equivalents may be resortedto, falling within the scope of the invention.

1. A semi-solid metal (SSM) casting process, comprising: providing ametal and a vertical die casting machine, heating the metal to a chosentemperature, cooling the metal for a determined period of time to form asemi-solid metal, wherein the time can be zero; and casting thesemi-solid metal in the vertical die casting machine.
 2. An SSM castingprocess according to claim 1, wherein the metal is an Al—Si alloy.
 3. AnSSM casting process according to claim 2, wherein Al—Si alloy is ahypereutectic Al—Si alloy comprising more than about 11.7 weight percentSi in Al.
 4. An SSM casting process according to claim 2, wherein Al—Sialloy is a hypoeutectic Al—Si alloy comprising less than about 11.7weight percent Si in Al.
 5. An SSM casting process according to claim 2,wherein Al—Si alloy is a 380 alloy.
 6. An SSM casting process accordingto claim 1, wherein the vertical die casting machine is an indexing typevertical die casting machine.
 7. An SSM casting process according toclaim 5, wherein the vertical die casting machine is a 1000 Ton ShuttleMachine.
 8. An SSM casting process according to claim 2, wherein thevertical die casting machine is an indexing type vertical die castingmachine comprising a shot sleeve that indexes between a pour station anda transfer station requiring an indexing time.
 9. An SSM casting processaccording to claim 5, wherein the temperature of metal is chosen suchthat the metal will form a semi-solid metal as it cools from indexingbetween the pour station to the transfer station..
 10. An SSM castingprocess according to claim 8, wherein the indexing time is chosen toachieve a determined rate of cooling.
 11. A means for SSM casting,comprising: providing a metal and a vertical die casting means, heatingthe metal to a chosen temperature, cooling the metal for a determinedperiod of time to form a semi-solid metal, wherein the time can be zero;and casting the semi-solid metal.
 12. A means for SSM casting accordingto claim 11, wherein the metal is an Al—Si alloy.
 13. An A means for SSMcasting according to claim 11, wherein the vertical die casting means isan indexing type vertical die casting means.
 14. A means for SSM castingaccording to claim 13, wherein the vertical die casting means is a 1000Ton Shuttle Machine.
 15. A means for SSM casting according to claim 11,wherein Al—Si alloy is a hypereutectic Al—Si alloy comprising more thanabout 11.7 weight percent Si in Al.
 16. A means for SSM castingaccording to claim 11, wherein Al—Si alloy is a hypoeutectic Al—Si alloycomprising less than about 11.7 weight percent Si in Al.
 17. A means forSSM casting according to claim 11, wherein Al—Si alloy is a 380 alloy.18. A means for SSM casting according to claim 11, wherein the verticaldie casting machine is an indexing type vertical die casting machinecomprising a shot sleeve that indexes between a pour station and atransfer station requiring an indexing time.
 19. A means for SSM castingaccording to claim 11, wherein the temperature of metal is chosen suchthat the metal will form a semi-solid metal as it cools from indexingbetween the pour station to the transfer station.
 20. A means for SSMcasting according to claim 11, wherein the chosen temperature is abovethe liquidus temperature of the metal.